The present invention is related in general to digital information transmission systems and, more particularly, to data recovery apparatus for use in a digital transmission system in which a high speed data stream is divided and transmitted synchronously on multiple parallel transmission media.
A continuing trend in the development of digital information transmission systems is to increase the bandwidth of transmission media by sending data at a faster data rate. Such transmission media include two-conductor unshielded cable, as can be utilized in telecommunication circuits. However, attempts to increase media bandwidth are restricted by physical limitations on the ability to increase the data transmission rate. For example, an increase in the data transmission rate is accompanied by an increase in the high frequency spectral content of the transmitted signal. Such an increase in the high frequency spectral content results in unacceptably high electromagnetic emission levels that are caused by common mode noise and line imbalance. Also, the levels of electromagnetic emissions resulting from transmission of digital information are subject to conformance with federal and international standards, for example the federal standard established by the Federal Communications Commission. Therefore, the data transmission rate is limited by these considerations.
One solution known in the art for transmitting data carried in a high data-rate data signal while minimizing the problem of high electromagnetic emissions, is the dividing of a high data-rate serial data stream to form multiple parallel data streams that are each transmitted at a correspondingly lower rate. For example, U.S. Pat. No. 4,630,286 discloses apparatus for dividing a single data stream having a data rate of 28.8 kilobits/second into two data streams, each having a data rate of 14.4 kilobits/second, that are then transmitted over two transmission lines.
In conventional systems in which a high data rate serial data stream is divided into multiple data streams for transmission on multiple parallel transmission media at a lower data rate, it is a typical practice to transmit the data on the parallel media in synchronism, such as by driving the transmissions of the two or more data streams in accordance with a single transmission clock signal. Such synchronized transmission is performed to facilitate the proper recovery of the data and the reformation of a single stream of data at the high data rate at the receiving end of the parallel transmission media. A problem experienced with dividing a high data rate data stream results from the different signal propagation speeds on the parallel transmission media, which cause the parallel data streams to fall out of synchronism, such that time skews develop between respective ones of the parallel data streams. These time skews hinder the successful reformation of the single high speed data stream from the parallel low data rate data streams at the receiving end of the parallel transmission media.
Techniques known in the art for reformation of a single high data rate data stream from multiple related streams of low data-rate data skewed in time, require the use of additional circuitry for removal of the time skew. In accordance with techniques known in the art, the reformation of the single high data-rate data stream requires the steps of recovering the data and then removing the skew associated with each one of the low data rate data streams. For example, it is known in the art to utilize separate phase locked loop circuits respectively dedicated to the multiple parallel transmission media to recover each of the multiple low data rate data streams. Then, separate circuitry is utilized to determine and remove the skew in those data streams so that the high data rate data stream can be reformed. Examples of circuitry for eliminating the time skew in each of two or more received data streams are disclosed in U.S. Pat. No. 4,818,995 and the above noted U.S. Pat. No. 4,630,286.
Disadvantageously, the need to provide additional circuitry to remove time skew from each of multiple low data rate data streams adds to both the complexity and cost of data recovery apparatus.